1. Field of the Invention
The present invention relates to a fuel cell. More particularly, the invention relates to a fuel cell with its cells disposed in a planar arrangement.
2. Description of the Related Art
A fuel cell is a device that generates electricity from hydrogen and oxygen so as to obtain highly efficient power generation. A principal feature of a fuel cell is its capacity for direct power generation which does not undergo a stage of thermal energy or kinetic energy as in conventional power generation. This presents such advantages as high power generation efficiency despite the small scale setup, reduced emission of nitrogen compounds and the like, and environmental friendliness on account of minimal noise or vibration. A fuel cell is capable of efficiently utilizing chemical energy in its fuel and, as such, environmentally friendly. Fuel cells are therefore envisaged as an energy supply system for the twenty-first century and have gained attention as a promising power generation system that can be used in a variety of applications including space applications, automobiles, mobile devices, and large and small scale power generation. Serious technical efforts are being made to develop practical fuel cells.
In particular, polymer electrolyte fuel cells feature lower operating temperature and higher output density than the other types of fuel cells. In recent years, therefore, the polymer electrolyte fuel cells have been emerging as a promising power source for mobile devices such as cell phones, notebook-size personal computers, PDAs, MP3 players, digital cameras, electronic dictionaries or electronic books. Well known as the polymer electrolyte fuel cells for mobile devices are planar fuel cells, which have a plurality of single cells arranged in a plane. As a conventional method for arranging a plurality of single cells in a plane, a base material (substrate) is used and a plurality of through-holes are provided in this base material which is a nonelectrolyte. And these through-holes are filled with electrolytes to fabricate planar fuel cells using a composite membrane. The use of the base material makes it possible to use an electrolyte whose proton conductivity is high but whose mechanical strength is weak. Also, the use of the base material reduces the electrolyte part as much as possible, thereby reducing the cost.
To reduce the size of the planar fuel cell, the interval (spacing) between cells must be narrowed. For example, a technique using a laser processing is known as a method for narrowing the interval between them. In this technique, a catalyst layer of normal membrane electrode assembly is partially removed by the use of laser beams so as to fabricate a planar fuel cell.
A resin film which is highly chemical-resistant and excels in the dimensional stability may be used as a material for the substrate.
As a fuel to be used for this type of fuel cells, hydrogen stored in a hydrogen storage alloy or a hydrogen cylinder, as well as methanol, is the subject of continuing investigations.
The amount of electric power generated by planar fuel cells varies depending on the total area of electrodes. As for the voltage, on the other hand, the voltage produced thereby varies depending on the number of cells connected in series. Accordingly, to reduce the size of the fuel cell while the required power and voltage are assured, the number of cells connected in series needs to be increased and the interval between adjacent single cells needs to be shortened. At the same time, as the spacing between the single cells gets smaller, the problem of possible short-circuiting between adjacent catalyst layers must be addressed.
To form single cells with a fine interval therebetween in the planar fuel cells using a composite membrane, the interval between the adjacent through-holes provided in the substrate needs to be as short as possible. Accordingly, the microfabrications of substrate and catalyst layers need to be performed. A method of laser processing, punching with a metallic mold or the like is used as the microfabrication of substrate. Though the laser processing proves useful when the microfabrication is performed on the catalyst layers, the substrate may be damaged when the catalyst layers are irradiated with laser and the substrate may sometimes be cracked. When any crack is caused, there is a problem where hydrogen leaks with the hydrogen used as a fuel and therefore the performance of the fuel cell degrades.
Though the leak of hydrogen may be prevented by increasing the thickness of the substrate, the thickness of an electrolyte filling the substrate increases, which causes a problem of degraded performance of the fuel cell. When the laser beams used for processing the catalyst layers transmit through the substrate with the laser irradiated from one side only, a catalyst layer formed in the back with the composite membrane interposed between the catalyst layers is also removed. This causes another problem, for instance, when the area of anode and cathode electrodes in the single cell is to be changed.